This application claims the benefit of Japanese Application No. 2000-063754, filed on Mar. 8, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a clutch control device of a continuously variable transmission provided in a vehicle.
2. Description of Related Art
In multistage automatic transmissions and continuously variable transmissions, a torque converter as an example of a power transmission means is provided between an engine and a transmission. The torque converter relieves a shift shock when the torque of the engine is inputted to the transmission when a vehicle is started or is rapidly accelerated.
At the interior of the torque converter, there is provided a hydraulic control type lockup clutch, which is capable of completely engaging an input shaft connected to an output shaft of the engine with an output shaft connected to an input shaft of the transmission in order prevent a slip between them. By completely engaging the lockup clutch, the engaged state of the input shaft and the output shaft of the torque converter can be switched from an engaged state caused by fluid friction of the torque converter to an engaged state caused by mechanical friction of the lockup clutch. This improves the fuel economy.
The above hydraulic control type lockup clutch controls the engagement state of the input shaft with the output shaft in the torque converter according to engaging force that is adjusted by pressing force, with which the lockup clutch is pressed by hydraulic pressure. While the lockup clutch is completely engaged, the hydraulic pressure for adjusting the pressing force against the lockup clutch is constantly set to the maximum value. The pressing force is set to such a high value as to prevent the slip of the lockup clutch.
If the lockup clutch is completely engaged in the state wherein a driver lifts a foot from an accelerator pedal to bring a throttle angle to zero (i.e., the lift-foot state), a shift shock occurs when the driver operates the accelerator pedal next time, and more particularly when the engine torque increases instantaneously, e.g., during a kick down shift. In order to avoid such a shift shock, the pressing force against the lockup clutch is adjusted in such a manner as to release the lockup clutch from the completely engaged state in the lift-foot state.
In the case of the multistage automatic transmission which change gears step by step, it is necessary to release the lockup clutch from the completely engaged state in order to avoid shift shock when the gears are changed. In the case of the continuously variable transmission, however, the gear ratios are changed continuously and it is therefore unnecessary to release the lockup clutch from the completely engaged state when the gear ratios are changed.
When the engine torque increases instantaneously e.g., during a kick down shift, the continuously variable transmission causes a shift shock as is the case with the multistage automatic transmission if the lockup clutch is completely engaged. To prevent such a shift shock, the continuously variable transmission preferably releases the lockup clutch from the completely engaged state.
The release of the lockup clutch from the completely engaged state, however, lowers the efficiency of the transmission and deteriorates the fuel economy. Accordingly, there is a desire to avoid the release of the lockup clutch from the completely engaged state.
It is therefore an object of the present invention to provide a clutch control device and method for use in a continuously variable transmission, which is able to improve the fuel economy and prevent a shift shock when there is a rapid increase in the engine torque.
The above object can be accomplished by providing a clutch control device for a continuously variable transmission, the clutch control device comprising: a clutch mechanism provided between an engine and the continuously variable transmission, the clutch mechanism being capable of variably engaging an output shaft of the engine with an input shaft of the continuously variable transmission from an unengaged state to a completely engaged state; a control unit for controlling an engagement state of the clutch mechanism; wherein the control unit has an engaging force setting part for setting a target engaging force according to engine torque; and the engaging force setting part sets a minimum engaging force capable of maintaining the completely engaged state of the clutch mechanism according to the engine torque and further sets the target engaging force to a value that is found by adding a predetermined value to the minimum engaging force.
The present invention can also be accomplished by providing a clutch control method for a continuously variable transmission, the clutch control method comprising the steps of: estimating engine torque from an engine operating state; setting a minimum engaging force capable of maintaining a completely engaged state of the clutch mechanism according to the estimated engine torque; setting a target engaging force of a clutch mechanism of the continuously variable transmission by adding a predetermined value to the minimum engaging force; and controlling the clutch mechanism according to that target engaging force.